Wireless devices such as terminals are also known as e.g., User Equipments (UE), terminals, mobiles, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed, e.g., between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which may be divided into cell areas, wherein each cell area may be served by an access node such as a base station, e.g., a Radio Base Station (RBS), which sometimes may be referred to as e.g., evolved NodeB “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g., macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The 3GPP Release 13 (Rel-13) feature “Licensed-Assisted Access” (LAA) allows an LTE equipment, such as a communication device, to also operate in the unlicensed 5 Gigahertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum may be used as a complement to the licensed spectrum. An Release 14 3GPP Rel-14 work item may add UL transmissions to LAA. Accordingly, communication devices may connect in the licensed spectrum, through e.g., a primary cell or PCell, and may use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum, through e.g., a secondary cell or SCell. Standalone operation of LTE in unlicensed spectrum may also be possible and is under development by the MuLTEfire Alliance.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) method may need to be applied. LBT may involve sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
LTE
LTE may use Orthogonal Frequency Division Multiplexing (OFDM) in the DL and Discrete Fourier Transform (DFT)-spread OFDM, also referred to as Single-Carrier Frequency Division Multiple-Access (SC-FDMA), in the UL. The basic LTE DL physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The UL subframe may have the same subcarrier spacing as the DL, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the DL. The subcarrier spacing has been chosen to be 15 kiloHertz (kHz), as shown.
In the time domain, LTE DL transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2, which illustrates the LTE time-domain structure. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame may range from 0 to 19. For normal cyclic prefix, one subframe may consist of 14 OFDM symbols. The duration of each symbol may be approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE may typically be described in terms of resource blocks, where a resource block corresponds to one slot, 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction, 1.0 ms, may be known as a resource block pair. Resource blocks may be numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions may be dynamically scheduled, i.e., in each subframe the base station may transmit control information about which terminals data is transmitted to, and upon which resource blocks the data is transmitted, in the current DL subframe. This control signaling may be typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe, and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The DL subframe may also contain common reference symbols, which may be known to the receiver, and used for coherent demodulation of e.g., the control information. A DL system with CFI=3 OFDM symbols as control region is illustrated in FIG. 3, which illustrates a normal DL subframe. The control region in FIG. 3 is shown as comprising control signaling, indicated by checkered squares, reference symbols, indicated by striped squares, and unused symbols, indicated by white squares. The reference symbols shown in the above FIG. 3 are Cell-specific Reference Symbols (CRS) and they may be used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Uplink transmissions may be dynamically scheduled, i.e., in each downlink subframe the base station may transmit control information about which terminals should transmit data to the eNB in subsequent subframes, and upon which resource blocks the data may be transmitted. The uplink resource grid may be comprised of data and uplink control information in the Physical Uplink Shared Channel (PUSCH), uplink control information in the Physical Uplink Control Channel (PUCCH), and various reference signals such as Demodulation Reference Signals (DMRS) and Sounding Reference Signals (SRS). DMRS may be used for coherent demodulation of PUSCH and PUCCH data, whereas SRS may not be associated with any data or control information but may be generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling. An example uplink subframe according to Rel-12 is shown in FIG. 4. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS may always be transmitted in the last symbol of a normal UL subframe. In FIG. 4, frequency multiplexing is used to separate the SRS from a first user (UE 1 SRS), from that from a second user (UE 2 SRS). The PUSCH DMRS may be transmitted once every slot for subframes with normal cyclic prefix, and may be located in the fourth and eleventh SC-FDMA symbols.
From LTE Rel-11 onwards, DL or UL resource assignments may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10, only the Physical Downlink Control Channel (PDCCH) may be available. Resource grants may be UE specific and may be indicated by scrambling the Downlink Control Information (DCI) Cyclic Redundancy Check (CRC) with the UE-specific Cell Radio Network Temporary Identifier (C-RNTI).
LTE PUCCH
In LTE, the PUCCH may be placed at the edges of the system bandwidth in the frequency domain. Each PUCCH transmission in one subframe may comprise a single RB at or near one edge of the system bandwidth in the first slot, followed by a second RB at or near the opposite edge of the system bandwidth in the next slot. Multiple UEs may be multiplexed onto the same PUCCH RBs, with user separation achieved via code division multiplexing in the frequency and/or time domain. The PUCCH may carry control information comprising Channel Quality Information (CQI), Hybrid Automatic Retransmission reQuest (HARQ) ACKnowledgment/Negative ACKnowledgment (ACK/NACK) and uplink scheduling requests. See 3GPP TS36.211 Section 5.4.
Carrier Aaaregation
The LTE Release 10 standard may support bandwidths larger than 20 MegaHertz (MHz). One important requirement on LTE Release 10 may be to assure backward compatibility with LTE Release 8. This may also include spectrum compatibility. That may imply that an LTE Release 10 carrier, wider than 20 MHz, may appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier may be referred to as a Component Carrier (CC). In particular, for early LTE Release 10 deployments, it may be expected that there may be a smaller number of LTE Release 10-capable terminals compared to many LTE legacy terminals. Therefore, it may be necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it may be possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this may be by means of Carrier Aggregation (CA). CA implies that an LTE Release 10 terminal may receive multiple CC, where the CC may have, or at least may have the possibility to have, the same structure as a Release 8 carrier. CA is illustrated in FIG. 5, where 5 carriers of 20 MHz each are aggregated to form a bandwidth of 100 MHz. A CA-capable communication device, such as a UE, may be assigned a Primary Cell (PCell) which is always activated, and one or more Secondary Cells (SCells), which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different. It may be noted that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more DL CCs UL CCs, even though the cell is configured with the same number of UL and DL CCs.
Wireless Local Area Network (WLAN)
In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) may be used for medium access. This means that the channel may be sensed to perform a Clear Channel Assessment (CCA), and a transmission may be initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission may be essentially deferred until the channel is deemed to be Idle.
A general illustration on an example of the Listen Before Talk (LBT) mechanism or process is shown in FIG. 6. After a Wi-Fi station (STA) A may transmit a data frame to a station B, station B transmits the ACK frame back to station A with a delay of 16 μs. Such an ACK frame may be transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station may defer for a duration of 34 μs, referred to as Distributed coordination function Interframe Space (DIFS), after the channel is observed to be occupied before assessing again whether the channel is occupied. Therefore, a station that wishes to transmit data may first perform a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station may assume that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station may wait for the medium to go idle, defer for DIFS, and wait for a further random backoff period.
In the above basic protocol, when the medium may become available, multiple Wi-Fi stations may be ready to transmit, which may result in collision. To reduce collisions, stations intending to transmit may select a random backoff counter and defer for that number of slots channel idle times. The random backoff counter may be selected as a random integer drawn from a uniform distribution over the interval of [0, Contention Window (CW)]. The default size of the random backoff contention window, CWmin, may be set in the IEEE specs. Note that collisions may still happen even under this random backoff protocol when there may be many stations contending for the channel access. Hence, to avoid recurring collisions, the backoff contention window size CW may be doubled whenever the station may detect a collision of its transmission up to a limit, CWmax, which may also be set in the IEEE specs. When a station succeeds in a data transmission without collision, it may reset its random backoff contention window size back to the default value CWmin.
Licensed-Assisted Access (LAA) in Unlicensed Spectrum
Up to now, the spectrum used by LTE may be dedicated to LTE. This may have the advantage that the LTE system may not need to care about the coexistence issue and the spectrum efficiency may be maximized. However, the spectrum allocated to LTE is limited, which may not meet the ever increasing demand for larger throughput from applications and/or services. Therefore. Rel-13 extended LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum may, by definition, be simultaneously used by multiple different technologies. Therefore, LTE may need to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum may seriously degrade the performance of Wi-Fi, as Wi-Fi may not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably may be to transmit particularly relevant control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a UE may be connected to a PCell in the licensed band and one or more SCells in the unlicensed band. Herein, a SCell in unlicensed spectrum is referred to as a Licensed-Access Secondary Cell (LA SCell) or Licensed-Assisted Access Cell. In the case of standalone operation as in MuLTEfire, no licensed cell is available for uplink control signal transmissions. FIG. 7 is a schematic representation illustrating LAA to unlicensed spectrum using LTE carrier aggregation.
The Maximum Channel Occupancy Time (MCOT) of a single DL+UL Transmission Opportunity (TXOP) in unlicensed bands may be limited by regional regulatory restrictions. For example, in Europe, EN Broadband Radio Access Networks (BRAN) is considering the following MCOT rules: a) Specify max TxOP=6 ms available for 100% of the time; b) Specify max TxOP=8 ms is available for 100% of time with a minimum pause of [TBD] μs, in order of hundreds of μs, after a maximum transmission of 6 ms; c) Specify max TxOP=10 ms is available for [TBD3]% of the time.
LBT in 3GPP LAA and MuLTEfire
In Rel-13 LAA, LBT for DL data transmissions may follow a random backoff procedure similar to that of Wi-Fi, with CW adjustments based on Hybrid Automatic Repeat reQuest (HARQ) Negative ACKnowledgment (NACK) feedback. Several aspects of UL LBT were discussed during Release 13. With regard to the framework of UL LBT, the discussion focused on the self-scheduling and cross-carrier scheduling scenarios. UL LBT may impose an additional LBT step for UL transmissions with self-scheduling, since the UL grant itself may require a DL LBT by the eNB. The UL LBT maximum CW size may then be limited to a very low value to overcome this drawback, if random backoff is adopted. Therefore, Release 13 LAA recommended that the UL LBT for self-scheduling should use either a single CCA duration of at least 25 μs, similar to a DL Discovery Reference Signal (DRS), or a random backoff scheme with a defer period of 25 μs including a defer duration of 16 us followed by one CCA slot, and a maximum contention window size chosen from X={3, 4, 5, 6, 7}. These options may be also applicable for cross-carrier scheduling of UL by another unlicensed SCell.
A short UL LBT procedure for the case involving cross-carrier scheduling by a licensed PCell remains open for further study. The other option which may be considered is a full-fledged random backoff procedure similar to that used by Wi-Fi STAtions (STAs).
Finally, the case of UL transmissions without LBT when an UL transmission burst follows a DL transmission burst on that respective carrier, with a gap of at most 16 μs between the two bursts, was left open for further study in Rel-14.
An example to illustrate UL LAA LBT and UL transmission when the UL grant is sent on an unlicensed carrier is provided in the schematic diagram of FIG. 8. As shown in the Figure, an eNB transmits an UL grant in a subframe n, in an unlicensed band. The UL grant is represented in the Figure with black rectangles in subframe n. The eNB first performs channel sensing with CCA. The UL grant is for a UE to transmit data in subframe n+4 in the unlicensed band. The UL data transmission is represented in the Figure with black rectangles in subframe n+4. Before starting UL data transmission, the UE also may need to perform channel sensing CCA.
PUCCH in MuLTEfire
Two forms of PUCCH transmission have been defined for MuLTEfire (MF): a short PUCCH (sPUCCH) comprising between two to six symbols in time, and a longer extended PUCCH (ePUCCH), which may span one subframe in time, as shown in FIG. 9. FIG. 9 is a schematic diagram illustrating examples of ePUCCH and sPUCCH within a TXOP in two different MF cells. In FIG. 9, a “U” is used to represent a full UL subframe. Any vertical rectangle of the same dimensions in the Figure represents a subframe. Any vertical rectangle of narrower breadth in the Figure represents a partial subframe. “D” is used to indicate DL. An LBT procedure is performed only once at the beginning the TXOP, followed by a DL burst. The sPUCCH may occur immediately after the Downlink Pilot TimeSlot (DwPTS) portion of a partial DL subframe as defined in Rel-13 LAA, while the ePUCCH may be multiplexed with PUSCH transmissions in 1 ms UL subframes. Both sPUCCH and ePUCCH may utilize an interlaced transmission mode based on Block-Interleaved Frequency Division Multiple Access (B-IFDMA). The UL sub-frame containing ePUCCH and PUSCH may also contain a Physical Random Access CHannel (PRACH) preamble. In other examples not depicted in FIG. 9, a TxOP may also be started from a UE side. In such other examples, the LBT may be performed by the UE and the TxOP may start with an UL burst.
Further details regarding any of the above may be found in 3GPP TS 36.211, V12.3.0 (2014-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, Release 12, 3GPP TS 36.213, V12.3.0 (2014-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures, Release 12, 3GPP TS 36.212, V12.6.0 (2015-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC), Release 12, and 3GPP TS 36.321, V12.1.0 (2014-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification, Release 12.
The usage of unlicensed spectrum may provide a way to increase the capacity of a wireless communications network to match an ever growing demand for transmissions. However, since the transmission in unlicensed spectrum is shared, and channel sensing procedures may need to be performed, transmissions may be significantly delayed. Furthermore, with existing methods, transmission in unlicensed spectrum of uplink control information in an uplink control channel, e.g., the LTE PUCCH, to a communication device supporting multiplexing of communication devices, may be unreliable, or unnecessarily delayed, as each of the multiplexed communication devices may need to perform a channel sensing procedure. Hence, while usage of unlicensed spectrum may provide a way to increase the capacity of a wireless communications network, the effectiveness of wireless communications in unlicensed spectrum is currently compromised.